Since the first report of phosphorescent organic light-emitting devices (PHOLEDs) by Forrest et al. in 1998, PHOLEDs have attracted much attention because they can achieve an internal quantum efficiency of 100%. Phosphorescent emitters typically have long lifetimes and diffusion lengths; however, concentration quenching and T1-T1 annihilation are normally major causes for the poor device performance when the emitter concentration is high. To solve these problems, PHOLEDs are always fabricated by doping the phosphorescent emitters into a suitable host material so as to reduce the emitter concentration. Therefore, the development of host materials is of extremely crucial for efficient electrophosphorescence.
An efficient host material should have a desirable bandgap for effective energy transfer to the guest, good carrier transporting properties for a balanced recombination of carriers in the emitting layer, energy-level matching with neighboring layers for effective charge injection, and decent thermal and morphological stabilities for extending the device's lifetime. Traditional host materials usually only have single carrier transporting property, like N,N′-dicarbazolyl-3,5-benzene (mCP) and 3-(4-biphenylyl)-4-phenyl-5-(4-tert-butylphenyl)-1,2,4-triazole (TAZ). They only have hole-transporting and electron-transporting properties, respectively. This unbalanced carrier transporting property of host materials have been shown to be detrimental to the turn-on voltage and stability of OLEDs. Thereby, recently bipolar host materials which can balance the carrier transports have aroused extensive interests.
Because bipolar molecule must contain both the p- and n-type groups, intramolecular donor-acceptor interaction will generally lower the triplet energy of the material. Therefore, recently reported bipolar hosts, like o-CzOXD and BUPH1, are mainly used in the green and red PHOLEDs. In contrast, bipolar hosts used in blue PHOLEDs are rarely reported. There are two possible regimes to obtain a bipolar blue host: either chemical groups which have extremely high triplet energies or a compound which the p-type group and n-type group are designed to be farther enough to decrease the intramolecular donor-acceptor interaction. However, the chemical groups with high triplet energies generally have small steric volumes. The bipolar compound might not have decent thermal and morphological stabilities.